In many types of thermal inkjet print heads the ink is fed from the reservoir to the ejection chambers through one or more slots made longitudinally in the inner part of the substrate, which is often a silicon chip. The ink flows from the rear substrate surface to the front surface, where the electronic as well as the microfluidic circuitries are realized. A single slot can feed one or two heater columns, which stay along the slot edges in the direction of the longitudinal chip axis.
Usually conductive, resistive, dielectric and protective thin films are deposed and patterned, to realize the circuitry. Possible devices like transistors, diodes, memories, etc. can be integrated in the circuitry using the semiconducting properties of the silicon.
The heaters are arranged in a plurality of longitudinal columns, which are adjacent to a through-slot, which is necessary for the ink feeding towards the ejection sites. It is possible to have either a single slot feeding two columns or several parallel slots feeding a corresponding number of column pairs.
So for example a polymeric layer is deposed onto the surface of the silicon chip and patterned to create the ejection chamber around each heater and the channels for the feeding with the ink flowing from the slot. Since the walls of the patterned profile act as an ink containing barrier, the polymeric layer is called «barrier layer».
A nozzle plate is assembled on the top of the barrier layer. It constitutes the ceiling of the ejection chamber and houses a plurality of nozzles, in one-to-one correspondence with the plurality of heaters. Therefore, also the nozzles are arranged in columnar arrays.
The structure created by the ink feeding slot, silicon chip, surface and ejection chambers and nozzles constitute the fluidic circuit of the print head.
In digital printing the ink is distributed onto the medium as a matrix array of dots arranged in rows and columns. The rows extend in the direction of the relative movement between print head and medium. The reciprocal of the distance between contiguous dots in a horizontal line (row) is the horizontal resolution. The reciprocal of the distance between contiguous dots in a vertical line (column) is the vertical resolution.
The vertical resolution is substantially depending on the distance between nozzles in the print head columns. The horizontal resolution is determined by the combination of the ejection repetition rate with the relative movement speed.
The growth of the ink bubble in a thermal print head is caused by a short current pulse applied to the heating resistor. A standard thermal print head has normally hundreds nozzles (up to more than one thousand). If all the nozzles would be activated at the same time, the total current flowing in the circuit would reach an excessive intensity (tens of Ampère). Such a high current level could damage the circuitry of the silicon chip, would require a very huge and expensive power supply in the printing station and the resulting noise might be troublesome.
To solve this issue it is necessary to avoid the general overlapping of the current pulses, i.e. only a subset of nozzles should be allowed to eject a drop at the same time. Therefore the plurality of nozzles in the print head can be divided in several subsets or «firing groups». For each group all the nozzles can be fired at the same time, the different groups are fired in sequence, with a programmed delay between one group and the next one.
In this way, the current pulses for activating all the print head nozzles are distributed in a larger time interval; the maximum current intensity in the device turns out to be equal to the current of a single heater multiplied by the number of heaters belonging to the same firing group.
Since the print head is moving with respect to the medium, it is necessary to stagger the different firing groups along the relative movement direction, according to their own activation timing.
Therefore, the plurality of nozzles in a column cannot be aligned with the vertical printing lines, because they are not activated together.
In FIG. 19 one possibility is shown for inclined linear column segments (blocks), vertically stacked; the nozzles belonging to the same firing group overlap the same vertical printing line.
As can be seen in FIG. 19, the slot profile is substantially a straight line and therefore the staggered heaters turn out to have different distances from the slot edge, depending on their own activation timing. Therefore the fluidic circuit of the nearest heater is shorter than the one of the most distant heater. The difference in the channel length gives a different fluidic behavior. The nearest heater turns out to be also the faster as it has the shortest refilling time, giving the maximum printing frequency. Due to the longer ink path, the rest of the heaters have a longer refilling time, depending on the distance from the slot, and thus they show a lower frequency. This spread limits the print head frequency to frequency of the slowest heater.
To compensate for this spread in the fluidic behavior of the ejection sites, suitable adjustments in the fluidic layout are necessary for each heater.
Document U.S. Pat. No. 8,714,710B2 suggests to produce a substantially equal path length for the fluid flowing from the feed channel towards the staggered resistors. This is achieved by a cantilever, which extends over the fluid channel. This is achieved by a thin film, which is removed in the central part leaving only the cantilever, followed by the completion of the process by removal of the silicon from the backside using laser and/or dry/wet etch. To realize a cantilever extending over the fluid channel, as described, soft etching methods are required on both wafer sides. This kind of process is suitable for a monolithic print head, where all the layers (including the nozzle plate) and all the holes or cavities are made through photolithographic processes.
U.S. Pat. No. 7,427,125 B1 suggests a wet etch process as a final step to complete to form a feed channel which adapts to the zigzag profile of the arranged resistors. By the wet etch process angled sidewalls are achieved. This wet etch process requires hard masks, which cannot be deposited onto e.g. the polymeric layer. Even if wet etching takes place only on the wafer backside, the resulting wall angle wouldn't fit with a layout with parallel slots, close to each other.